The average age of aircraft in use has continued to increase. Both the private sector and government are retaining aircraft for a longer time period before replacing the aircraft. The decision to fly and support aircraft well beyond their original design life has required an increased focus on inspection, maintenance, and repair, and the cost associated with those items.
With the extended life of aircraft, there is an increasing concern in maintaining an accurate assessment of the condition of the aircraft. One of the concerns relates to the deterioration of the structure of the aircraft, including frames, bulkheads, ribs, spars, mounting pylons and skin. Those structures are subject to deterioration from influences such as corrosion or fatigue. Fatigue is the gradual deterioration of a material which is subjected to repeated loads.
A common method of determining the condition of the structure is to monitor the progression of cracks. One technique is a liquid penetrant test where the coating is removed and the structure is covered with a liquid penetrant dye to see what cracks have developed. This technique is capable of detecting cracks that are greater than 0.003 inches in depth. In using this technique it is assumed that there are cracks that are just smaller than cracks that are detectable. With this assumption, the monitoring is at such intervals that a crack just under the size detected and growing at a predicted rate would be detected before there is major damage. A safety factor is added such that the monitoring interval is one half this time period.
An alternative method of detection uses conventional eddy current sensors that can detect discrete individual cracks but are not well suited for detection of microcrack clusters.
This invention relates to an apparatus and a method of detecting wide spread fatigue damage (WFD) on aircraft. It is desired to have a method of monitoring cracks on structural components, including skin panels, of aircraft to determine the condition of the aircraft. With an accurate representation of the condition of the aircraft, its use and maintenance can be tailored.
A meandering winding magnetometer (MWM) having a plurality of parallel spaced linear conductor elements is placed in proximity to the aircraft. An electromagnetic field is imposed on the aircraft and the resulting response is sensed. The response is transformed to determine the conductivity of the aircraft structure.
Mapping of the conductivity of the aircraft structure produces an indication where microcracks are located in the structure. Early indications of the density, spatial distribution and spatial orientation, as well as the size, of the microcracks give the user an indication of WFD. The microcracks determined using this method are below those detected by conventional non-destructive testing (NDT) techniques, such as eddy current sensing, and they typically occur in microcrack clusters.
The method uses one or more of several factor to identify the onset of WFD and the presence of distributed microcracks. These factors include:
1. The absolute conductivity image from the MWM shows distinct spatial variations with regions of reduced conductivity of over a certain length.
2. The absolute conductivity is lower at the surface than in the core. Surface coatings must be taken into account when examining this factor.
3. The spatial variations in conductivity along the surface and as a function of depth from the surface are consistent with loading of the aircraft. The variations may be consistent with possible high and low cycle fatigue loading induced damage of the structure.
In a preferred embodiment, the electromagnetic field is measured at varying frequencies. Higher frequencies are used to determine the thickness of any surface coatings such as Alclad.
In one embodiment, the MWM sensor is mounted on the aircraft using a flexible adhesive in a location which is not readily accessible to personnel. The sensor can be used for on-line monitoring of fatigue. In the same or other embodiment, the MWM sensor can be mounted on a complex curved shape.
The meandering winding magnetometers MWM sensor can be configured into an array for high resolution surface imaging and small crack detection. The models account for any cross talk so that the elimination of cross talk between sensing elements is not required. The grid methods automatically compensate for lift-off variation at each sensing element, therefore not requiring consistent lift-off control over the array footprint.
The grid measurement approach accounts for curvature of grid lines (lift-off and other property lines) as well as nonlinear variations in the sensitivity of the sensor response to variations in relevant properties to improve lift-off compensation.